For three decades, policy studies of health and health care in the United States have consistently called for strengthening primary care delivery as a means of addressing the tripartite concerns of cost control, quality assurance, and improved access. Yet, limitations of previous measures leave substantial uncertainty about the extent to which primary care is currently achieved and the extent to which strategies for expanding it are succeeding. This study introduces a methodology for measuring each of the dimensions that are widely viewed as distinguishing and defining characteristics of primary care: accessibility, continuity, comprehensiveness and integration. As such, it forms the basis for delivery system comparisons of primary care performance. Data on each primary care dimension will be obtained through a self- administered questionnaire, fielded to 10,200 adult subscribers (18 years and older) in 12 health care plans in Massachusetts. Study participants will be randomly drawn from the roster of active adult employees of the Commonwealth of Massachusetts (n=81,267), stratified by health plan. Represented among the 12 health plans are each of 6 delivery systems that dominate the current US health insurance market: indemnity insurance, preferred provider organization (PPO), staff-model health maintenance organization (HMO), group-model HMO, network-model HMO, and independent practice association (IPA). In addition, to identify organizational features that are probably invisible to patients but might influence primary care delivery, the study includes 120 ninety-minute interviews with the executive leadership and physicians from each plan. Data from subscribers, physicians and plan executives will be- used in multivariate analyses of primary care performance. Seven measures of the 4 distinguishing dimensions of primary care, derived from items on the subscriber survey, form the set of dependent variables for analysis. The analytic methods will identify primary care performance differences within and among indemnity, PPO and HMO systems, controlling for differences in patient and physician profiles. Information obtained from plan executives and physicians will highlight specific organizational features that contribute to delivery system performance differences. The study will yield information unequaled in scope regarding the characteristics of primary care received by a population of insured working adults. Evidence of delivery system differences in primary care performance will suggest public and private sector approaches through which to assure and expand primary care. The methodology will be applicable nationwide for purposes of measuring, monitoring and improving primary care.